It is known that a work hardened to different depths will have a reduced strength. It is essential to harden works to an even depth over the entire surfaces. Many proposals have been made; two of the examples are disclosed in Japanese Patents No. 3,499,486 (Reference 1) and No. 3,522,636 (Reference 2).
The References (1) and (2) disclose an apparatus designed to harden a plurality of works, such as cams, mounted on a shaft at different angles, so as to harden their surfaces simultaneously. The apparatus disclosed there have the same structure which shares the feature of hardening works in that the oppositely located two heaters are eccentrically rotated by a single power source. Each heater is provided with a bearing having an eccentric cam, which is connected to the power source by means of a timing belt so as to effect the simultaneous heating.
However, a disadvantage is that the above-mentioned apparatus must require many component parts, which increase the production cost and a relatively large site for installation.
In order to solve the problems, the inventors of the present invention invented a hardening apparatus shown in FIG. 1, having a mount 50 including a heating coil 55. The mount 50 is provided with two bearings 51 and 52 each having their own eccentric cams 53 and 54 driven by shafts 56 and 57, respectively. The shafts 56 and 57 are synchronously rotated. By rotating the two shafts 56 and 57, the heating coil 55 can eccentrically rotate in the same plane as a work 60. While the heating coil 55 is in the eccentric rotation, the work 60 is heated and hardened.
The apparatus shown in FIG. 1 has solved the problems of the known apparatus disclosed in the above-mentioned two literatures (1) and (2), but the difficulty is that the two shafts 56 and 57 are required to rotate in exact synchronism with each other. In order to achieve it, the cams must be made to high precision, thereby increasing the production cost and consuming time. In addition, it is difficult to continue to keep the two shafts 56 and 57 rotating in precise synchronism. Even if a small differentiation occurs between them, the heating coil fails to rotate eccentrically.
Therefore, it is an objective of the present invention to reduce the number of component parts and the area of the installation site, and also to ensure easy maintenance of the hardening apparatus.